Many human breast tumors contain estrogen receptors and many that do are responsive to hormone therapy. The growth of cells from estrogen dependent human and animal tumors in vitro, however, is not estrogen dependent nor, in most cases, estrogen responsive. We propose to test the hypothesis that the role of estrogen in breast cancer is to act not as a mitogen, but rather to induce in the cells properties essential or related to the malignant state, including not only tumor growth but invasion and metastasis. We will use the human breast cancer cell line MCF-7 as a model system for these studies. MCF-7 requires estrogen in order to form tumors in nude mice but is not dependent on estrogen for growth in culture. We have shown that estrogen at physiological concentrations stimulates the production of plasminogen activator by MCF-7 cells under conditions where the growth rate is unaffected by estrogen. We have clones of MCF-7 which differ in morphology, resistance to the antiestrogen tamoxifen and ability to produce plasminogen activator. We propose to study the effect of hormones including estrogen, progesterone, insulin and prolactin on the properties of these clones in vitro. We will measure properties which have been considered as possible markers of transformed cells including: Colony formation in soft agar, reactivity to concanavalin A, production of plasminogen activator, morphology and growth pattern in collagen-coated sponge cultures. We will also measure resistance to the antiestrogen tamoxifen and the levels of estrogen and progesterone receptors and will test the effect of hormones on these clones in an in vitro assay for tumor cell invasion. The in vitro results will be correlated with the ability and need for estrogen of the clones to form tumors, invade and metastasize in nude mice. In this way, we will attempt to define those cell properties related to tumor formation by breast cancer cells and the role of hormones, especially estrogen, in the process.